The invention relates to a monitoring circuit for at least one supply voltage, including a first threshold switch, which if the supply voltage drops below a predeterminable threshold value generates a first warning signal USWN, and a second threshold switch, which after a fixed warning period tw elapses generates a second warning signal TOTUSN.
Such a monitoring circuit is known (German Patent 39 10 212 C2). In this monitoring circuit, the first and second warning signals and a signal from the monitoring circuit, which responds if the supply voltage drops below a predetermined lower threshold, put switch elements in a switching state in which terminals for the warning signals are disconnected from the pole furnishing the supply voltage at a high voltage level and are connected to the other pole that furnishes the low supply voltage. This assures that the first and second warning signals have the requisite level for reliable detection even if the supply voltage, for a relatively long period of time, is at a level which while somewhat high is still not adequate for operating a data processor.